mapping not honoring document_type

I set a document_type directive.
Then use Influencer13.create_index!
But the mapping does not contain my document type.
Instead it contains a type derived from class name.

``` ruby
class Influencer13
  include Elasticsearch::Persistence::Model
  index_name "influencers"
  document_type 'influencer'
  mapping do
    puts @type
    @type = 'influencer'
  end
end
```

=> "influencer13"
